A mash seam welder employs a welding method as below. Two metal plates are placed to overlap each other at their end portions. The overlapping portions are pressed by a pair of electrode wheels and continuously welded together by applying a welding current thereto. At the same time, the joint portion softened by being heated to high temperature is rolled by the electrode wheels to reduce the thickness of the joint portion. However, this welding method cannot roll the joint portion to a level corresponding to the base material thickness. There is a problem in that the thickness of the joint portion is increased to approximately 120% to 160% of the thickness of the base material (the metal plate), which forms a step between the joint portion and the base material.
The following method is proposed as a method of reducing the thickness of the joint portion. A pair of pressure rollers is installed adjacently to and on one side of a pair of upper and lower electrode wheels. Before mash seam welding, the overlapping portions of the metal plates are rolled by the pressure rollers by moving a base frame supporting the electrode wheels and the pressure rollers in a pressure roller preceding direction. After the completion of the rolling, the metal plates overlapping each other are made slightly away from each other to reduce the thickness of the joint portion. Thereafter, the overlapping portion is pressed by the electrode wheels by moving the base frame in the opposite direction and is continuously subjected to mash seam welding by applying welding current thereto. Further, the joint portion is rolled by the pressure rollers to reduce its thickness. See Patent Documents 1 and 2.
In order to reduce the thickness of a joint portion subjected to mash seam welding and to reduce tact time, a method and apparatus are proposed as below. First and second pairs of upper and lower pressure rollers are installed adjacently to and on both sides of a pair of upper and lower electrode wheels. Overlapping portions of metal plates are rolled by the first pressure rollers by moving a base frame supporting the electrode wheels and the pressure rollers in one first roller preceding direction. Thereafter, the rolled portion is pressed by the pair of electrode wheels following the pressure rollers and is continuously subjected to mash seam welding by applying welding current thereto. Further, the joint portion is rolled by the second pressure rollers following the electrode wheels to reduce its thickness. See Patent Document 1 and Patent Document 2.
Patent Document 1: JP-2-15314-B
Patent Document 2: JP-2-16191-B